The present invention relates to membrane filters and more particularly to membrane filters that have predetermined controlled porosity.
Membrane filters are typically microporous films made from a variety of materials such as, for example, polypropylene, polysulfone, PTFE (polytetrafluoroethylene), nylon, etc. They are used to retain particles or microorganisms larger than their stated pore size by surface capture and some particles smaller than their stated pore size by other mechanisms. Such filters usually have a rated pore size within the range of about 0.1 to 5 microns. The thickness of these filters is generally about 150 microns.
There are many different types of filters available for a variety of filtering purposes. The filters typically vary in separation capability, adaptability to the gas or liquid being filtered and the filtering environment. The choice of filter for a specific filtering application is mainly based upon pore size and membrane material. The pore size rating will determine the size of particles or microorganisms that can be separated. The membrane material will determine the type of liquid or gas which can be filtered and the relative controllability of pore size.
Typically, pore sizes are rated as nominal or absolute at particular sizes. All filters are rated at a particle size range over which they will retain in an absolute manner, and then all particles smaller than this will be retained in a nominal manner. For example, a filter may be absolute rated at 5 microns, but have only nominal retention of particles less than 5 microns in diameter.
Nominal retention depends upon adsorption of particles onto filter surfaces as the major mechanism of retention. This mechanism depends on particles significantly smaller than the filter's true pore finding size adsorption sites during their passage through the filter media. Particles are retained by electrostatic attraction or other surface-dependent phenomena. Retention can therefore be grossly affected by test and use parameters which can act either to dislodge particles, reduce contact time between particles and media, or saturate the adsorption sites.
Absolute retention ratings are based solely on mechanical capture, in which all particles equal to or greater than the rated pore size are physically retained within the filter because of its pore dimensions. Theoretically, all the pores in such filters are smaller than the rated pore size.
Although it is possible to predict with reasonable accuracy the maximum size of the largest significant number of pores in these membranes, a direct method of determining pore size is not available. There is a typical kind of bell curve that describes the distribution of pore size in a given product. Depending upon the product, pore size is relatively easy to control within certain ranges, but very difficult to control outside of these ranges. Since pore size is extremely important in separating desired particles from a liquid or gas stream, the ability to directly and accurately determine pore size would be a great advantage. The reason pore size cannot be directly determined is due to the common methods of manufacturing membrane filters. The most common methods are phase inversion process, skiving followed by biaxial stretching and nuclear bombardment. Each of these processes forms pores in the membrane, but these pores are irregularly shaped and distributed throughout the membrane with individual pores having several different dimensions between the top and bottom of the membrane filter.
The phase inversion process for fabricating membrane filters involves the use of three ingredients. A polymer is dissolved in a solvent, to which is added a relatively insoluble pore former. This mixture is cast onto a belt or glass plate, and the solvent is driven off. The pore former is trapped in the polymer as the polymer comes out of solution. Pore former trapped in spheres of polymer are called micells. As the number of micells increases (with decreasing solvent), they grow together to from an open-cell structure. Thereafter, the pore former is extracted, leaving the pure polymer as an open-cell structure. In the phase inversion process, time and temperature are important in determining pore size of the membrane filter. In this type of membrane filter, pore size is easy to control where the desired size is less than .5 microns, but very difficult to control in larger pore sizes, because the bell curve becomes very wide.
There are numerous limitations on the use of phase inversion-type membranes. As just described, one such limitation is the controllability of larger pore sizes. A second limitation is that the finished membrane filter may be readily attacked by a variety of solvents, which greatly restricts its field of use. Because the formation of these membranes relies upon the use and effect of solvents on the polymer, they cannot be used in certain commercially significant filtration processes utilizing solvents, such as, for example, production of antibiotics and solid-state chips.
The solvent problem found in phase inversion-type membrane filters is overcome by the use of membrane filters made from PTFE (polytetrafluoroethylene). This type of membrane filter has been developed by W.L. Gore Company and goes by the tradename of "Gore-Tex." PTFE is an inert material not vulnerable to attack by solvents. PTFE is a sintered compression-bonded amalgam. Membrane filters are made from this material by first skiving a billet of the material, that is, peeling off a very thin sheet from the billet, followed by biaxial stretching of the membrane to cause a type of structural failure in the membrane which results in the formation of pores. These pores are interconnected by very fine tendrils which affect pore size. When a stream of gas or liquid is passed through the membrane, the interconnecting tendrils capture the particles.
Although PTFE-type membrane filters avoid the problem of solvents, they nevertheless have limitations. First, practical unsupported membranes of such materials cannot be made with pore sizes greater than 1 micron, because the additional stretching necessary to open the pore size reduces the thickness of the membrane, and the product becomes too thin for handling. Additionally, as the membrane is further stretched, the tendrils begin to break, which greatly and randomly increases pore size beyond what is desired. A second disadvantage of these membranes is that they are attacked by radiation. Radiation breaks the weak bond chain that exists in PTFE materials. The type of bonding is not atomic or molecular, but rather sintered compression bonding. This is a serious disadvantage because medical and pharmaceutical applications of these filters rely upon radiation as the preferred method of sterilization.
A third method for formation of membrane-type filters is nuclear bombardment, which is used by the Nuclepore Company. A thin sheet of polymer is exposed to nuclear bombardment to weaken the structure by carbonizing small spots. These spots are then etched by a material which attacks only carbon. The resulting burn holes define the pore. Diameters are generally limited to less than 1 micron.
One disadvantage of membrane filters formed from nuclear bombardment is that it is not possible to get a large number of pores per unit area, and therefore these membranes have a low flow-through rate. Another disadvantage is that this process cannot be used on PTFE material because of the inherent susceptibility of PTFE to radiation. A further disadvantage of this type of membrane filter is the inability to accurately control the pore size, a common disadvantage of all of the above membrane filters. In nuclear bombardment-type membrane filter formation, double hits can occur at a specific site, creating a pore which is substantially larger than desired.
To summarize, all of the above membrane filters have various disadvantages dependent upon the application to which they are to be employed. Common disadvantages of current membrane filters are the inability to accurately predict pore size, poor chemical compatibility, inability to make large pore sizes and inability to obtain a membrane filter with controlled porosity.